For the same reason they outlawed it in their own territories to begin with: Because it was evil.
I think we can agree that it had nothing to do with caring about the slaves.
Uh, no, I don't think we can.
I was thinking that 4 million sudden extra bodies in the poor southern economy would have the same effect as high immigration, keeping the wages of the poorest workers suppressed and it would keep the South from developing economically, while the North would benefit from their ownership of Southern industries.
Sudden extra bodies? Those bodies were already there. How much in wages were the slaves paid? The whole point of slavery was to have forced involuntary labor you didn't have to pay one cent to for their work. That obviously drives down and even eliminates the concept of a labor market and wages. Your comments literally contradict themselves.
And slavery made the South poorer and kept it from developing economically - greatly reduced commerce, stifled innovation. Slavery by definition is not free market and certainly don't provide for a labor market. The North vastly outproduced the South on everything including agriculture - innovation in technology and paid workers greatly increased economic activity and tax bases. One of the big reasons the North won the war. Common sense.
Southern slaves were paid. They were provided with food, clothing and shelter; perhaps meager, but not free to their owners. They were cared for when they were young and when they were old, again not free; and they were given Sundays off.
ML/NJ